The object of the present invention are infusible polyamide-imide powders whose very fine grains have a spheroidal form.
By polyamide-imides, we mean infusible (unmeltable) polymers (because they possess a three-dimensional lattice) resulting from the reaction of polyisocyanates with polyamides or copolyamides of molecular weight equal to or above 200.
Certain ones of these polyamide-imides have already been described. See particularly PATENTSCHRIFT No. 750,427 published in Germany May 25, 1944.
In order to make such polyamide-imide powders, the standard methods for the manufacture of polyamide powders cannot be used.
Those procedures utilizing a solution and precipitation by a non-solvent or by lowering of the temperature are inoperative since these polyamide-imides are insoluble in solvents because of their three-dimensional lattice.
Such powders also cannot be obtained by chemical conversion of very fine polyamide powders of spheriodal form (for instance, those obtained in French Patent Application No. 8501274) since if, for instance, one would cause a polyisocyanate to react with them at moderate temperatures, there would be a conversion into polyamide-imide only superficially and, on fusion (melting) there would be destruction of the spheriodal form.
Moreover, the crushing, either at ordinary temperature or cryogenically, of pieces of polyamide-imide leads to a coarse powder whose irregular pieces have the form of fragments (chips) whereas the powders which we claim have a very regular shape; they are in fact composed of very fine grains of spheroidal shape whose diameter is between 2 to 40 microns.
The literature describes fabrication procedures of polyamide powders by the anionic polymerization of lactams in solution (French Pat. Nos. 1,601,194, 1,601,195, 1,602,751, French Patent Applications Nos. 8501274 and 8504739), but all of the powders thus obtained are polyamide or copolyamide powders (and not polyamide-imide powders) which are fusible (meltable) and soluble in solvents such as m-cresol, hexafluoroisopropanol and the like.